PROJECT SUMMARY/ABSTRACT Autoimmune thyroiditis (AIT) affects several million people in the U.S., and many more worldwide. Although much of the pathophysiology of AIT is understood, and various treatment protocols are available, the events that trigger the autoimmune process remain sketchy. Knowledge of that is essential in order to fully understand how AIT is initiate and perpetuate. It has been known for over three decades that thyroid stimulating hormone (TSH) is produced by cells of the immune system; however, the significance of that remains elusive. Our laboratory identified a novel splice variant of the ?-subunit of TSH (TSH?v), the first alternatively spliced form of TSH? to be identified in mice and humans. Of particular interest, TSH?v is the only form of TSH? produced by the immune system. We have speculated that under normal physiological conditions immune system-derived TSH?v may play a role in maintaining the homeostatic balance in thyroid hormone output and host metabolic activity, but that the continual presence of TSH?v-producing leukocytes in the thyroid may be a causative factor in the development of thyroiditis. Support for this comes from findings from our laboratory in which splenic leukocytes traffic to the thyroid and secrete TSH?v during bacterial and viral infection, from preliminary studies showing increased intrathyroidal TSH?v expression in a murine model of AIT, and from studies by others demonstrating an increase in TSH?v expression in patients with Hashimoto?s thyroiditis, a form of AIT. The hypothesis to be tested is that intrathyroidal production of immune system TSH?v is a causative factor in the pathogenesis of AIT, and that suppression of immune system TSH?v in mice will prevent or significantly ameliorate AIT. This will be done using an established animal model of AIT (NOD.H2h4 mice) through two specific aims. Aim 1: To identify the source of immune system TSH?v in mice with AIT. Aim 2: To knockdown TSH?v in vivo in cells of the immune system to assess its involvement in AIT. If TSH?v is found to be a factor in driving AIT, it will provide new perspectives from which to mechanistically understand the inflammatory process in AIT in ways that have not been realized to the present. A better understanding of that could have significant heuristic benefits for developing novel patient treatment protocols.